Modern complex envelope modulation schemes such as those used in Enhanced Data rates for GSM Evolution (EDGE), Wideband Code Division Multiple Access (WCDMA), Bluetooth-Enhanced Data Rate (BT-EDR), Wireless Local Area Network (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), etc. impose strict performance requirements on transceivers developed to support them, especially wireless handset transmitters. Stringent performance requirements for many aspects of polar transmitters exist as well. A circuit diagram illustrating an example prior art polar transmitter employing complex modulation based on direct phase and amplitude modulation is shown in FIG. 1. The circuit, generally referenced 10, comprises a coder 12, I and Q TX filters 14, 16, polar coordinate converter 18, local oscillator 20 and multiplier 22.
In operation, the bits bk to be transmitted are input to the coder, which functions to generate I (real) and Q (imaginary) symbols therefrom according to the targeted communications standard. The I and Q symbols are pulse-shaped and the resulting baseband signals are converted to phase (Ang{s(t)}), and magnitude (Mag{s(t)}) baseband signals by the polar coordinate converter 18. The phase data is used to control the local oscillator 20 to generate the appropriate frequency signal, which is multiplied in multiplier/mixer 22 by the magnitude data resulting in the output RF signal x(t). It is noted that this polar modulation scheme is better suited for digital implementation rather than analog implementation.
Linearization of an RF power amplifier (PA) and associated transmitter for complex modulation schemes, including EDGE, WCDMA, WLAN etc. is a daunting task. The varying amplitude of these modern transmission standards cause both amplitude (AM/AM) and phase (AM/PM) domain distortions which can potentially impact key transmitter parameters such as the signal constellation and the close-in modulation spectral mask. The distortions also cause spectral re-growth which results in reduced isolation with adjacent channels, i.e. adjacent channel power ratio (ACPR), adjacent channel leakage ratio (ACLR), etc. as well as causing increased noise due to system non-linearity. As a result, the effort to improve linearization is at the forefront of the modern cell phone design, as it is motivated by a need for higher power-added efficiency (PAE) and lower dissipated power resulting in improved cell phone battery life. Co-linearization of the RF power amplifier and associated transmitter results in superior transmitted signal characteristics causing a reduction of the overall bit error rate (BER) of the communication system. This results in robust wireless call quality and a reduction in the probability of calls being dropped.
A number of modern spectrally efficient enhanced data rate modulation techniques use both amplitude and phase/frequency modulations. Due to the large envelope fluctuations that are possible, such modulation schemes place additional linearity constraints on the transmitter devices. Transmitting modulated signals with high peak to average power ratio (PAR) through nonlinear devices causes undesired spectral re-growth and increases the resulting bit error rate (BER). Linearization of the power amplifiers within the transmitters is therefore required to meet the spectral requirements of many wireless standards. In addition, the nonlinear characteristics of such devices are known to vary significantly within the span of their lifetime due to temperature and voltage changes or aging of the device itself.
For RF power amplifiers, any amplitude, phase and time distortions can have a detrimental effect on the TX system performance. In the spectral domain, phase distortion of a complex signal can manifest itself in adjacent channel leakage (ACL) or adjacent channel leakage ratio (ACLR). In a complex vector domain, amplitude and phase distortions affect the complex modulation vector. This distortion is expressed as the error vector magnitude (EVM). All physical amplifiers contribute distortions such as random noise, phase noise, amplitude compression (AM/AM) and amplitude induced phase/delay variation (AM/PM). RF amplifiers used near the output of a cellular TX chain generate relatively smaller output power (i.e. <5 W) and are dominated by AM/AM and AM/PM distortion in their steady-state operation.
Linearization techniques to compensate for such distortion are known in the art as they are currently an area of extensive research in both academia and industry. Most prior art linearization methods fall into one of two categories: either open loop methods or closed loop methods. Both open and closed loop methods have advantages and disadvantages but neither on its own is satisfactory to be used in inexpensive and mass produced single chip radios.
One prior art open loop approach requires intensive characterization of the nonlinear elements across all contributing variables including process, voltage and temperature (PVT), aging, frequency of operation, output power-level and antenna load variations. Such characterization, however, is very time and resource consuming and is not sufficiently robust. In addition, the factory calibration time is quite long and expensive due to the need for a dedicated test bench, external test equipment and test time. Furthermore, this scheme typically requires, for example, extensive temperature characterization for possible compensation during regular operation. This creates the dilemma that the on-chip temperature measurements may not be accurate and may not be representative of the PA operating temperature. Moreover, this scheme offers no reprieve from the voltage standing wave ratio (VSWR) variations due to impedance mismatch variations between the PA output and the antenna typically caused by variations in the antenna surroundings.
One prior art closed loop predistortion method has the disadvantage of requiring sophisticated and very well balanced analog circuitry for wide bandwidth loops of orthogonal phase modulation (PM) and amplitude modulation (AM) paths. Not only does this scheme consume relatively large amounts of power, but it also is subject to failure in face of large interferers that appear at the antenna port. Furthermore, the precise analog delay and gain balancing required takes its toll on the overall yield that can be achieved by the scheme.
Unfortunately, both of the prior art linearization techniques referred to above (i.e. both open and closed loop) are not suitable for use with the low cost GSM/EDGE/3G cellular market targeted by the DRP™ based fully integrated cellular radios. These prior art linearization techniques necessitate the use of expensive test equipment and typically result in a disproportionate amount of test time for linearization in contrast to other transmitter tests. Further, these prior art approaches to linearization are not amenable to (1) ultra low cost factory testing using Very Low Cost Tester (VLCT) equipment which typically has limited RF stimulus and capture capabilities or to (2) taking advantage of built-in self test (BIST) techniques used for self calibration and automated (device and mode) failure testing.
Therefore, in general, there is a need for a linearization mechanism capable of linearization of an RF power amplifier and associated transmitter for complex modulation schemes that overcomes the disadvantages of the prior art schemes. The linearization scheme should be capable of operation in a DRP based single chip radio whereby it can take advantage of on-chip DRP resources to achieve efficient linearization of the power amplifier.